The Effects of Individual Characteristics of the Naval Personnel on Sleepiness and Stress during Two Different Watchkeeping Schedules

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ The Effects of Individual Characteristics of the Naval Personnel on Sleepiness and Stress during Two Different Watchkeeping Schedules © 2022 by the authors Published version Myllylä, Mikko; Kyröläinen, Heikki; Ojanen, Tommi; Ruohola, Juha-Petri; Heinonen, Olli J.; Simola, Petteri; Vahlberg, Tero; Parkkola, Kai I. Myllylä, M., Kyröläinen, H., Ojanen, T., Ruohola, J.-P., Heinonen, O. J., Simola, P., Vahlberg, T., & Parkkola, K. I. (2022). The Effects of Individual Characteristics of the Naval Personnel on Sleepiness and Stress during Two Different Watchkeeping Schedules. International Journal of Environmental Research and Public Health, 19(20), Article 13451. https://doi.org/10.3390/ijerph192013451 2022 Citation: Myllylä, M.; Kyröläinen, H.; Ojanen, T.; Ruohola, J.-P.; Heinonen, O.J.; Simola, P.; Vahlberg, T.; Parkkola, K.I. The Effects of Individual Characteristics of the Naval Personnel on Sleepiness and Stress during Two Different Watchkeeping Schedules. Int. J. Environ. Res. Public Health 2022,19, 13451. https:// doi.org/10.3390/ijerph192013451 Academic Editors: Annina Ropponen, Anette Harris and Kati Karhula Received: 10 September 2022 Accepted: 15 October 2022 Published: 18 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Environmental Research and Public Health Article The Effects of Individual Characteristics of the Naval Personnel on Sleepiness and Stress during Two Different Watchkeeping Schedules Mikko Myllylä 1,2,*, Heikki Kyröläinen 3,4 , Tommi Ojanen 5, Juha-Petri Ruohola 6, Olli J. Heinonen 7, Petteri Simola 5, Tero Vahlberg 8and Kai I. Parkkola 4,9 1Centre for Military Medicine, The Finnish Defence Forces, 20241 Turku, Finland 2Doctoral Programme in Clinical Research, University of Turku, 20014 Turku, Finland 3Neuromuscular Research Center, Faculty of Sport and Health Sciences, University of Jyväskylä, 40014 Jyväskylä, Finland 4Department of Leadership and Military Pedagogy, National Defence University, 00861 Helsinki, Finland 5Human Performance Division, Finnish Defence Research Agency, The Finnish Defence Forces, 04310 Tuusula, Finland 6The Navy Command Finland, The Finnish Defence Forces, 20811 Turku, Finland 7Paavo Nurmi Centre & Unit of Health and Physical Activity, University of Turku, 20520 Turku, Finland 8Department of Biostatistics, University of Turku, 20014 Turku, Finland 9Faculty of Medicine and Health Technology, Tampere University, 33100 Tampere, Finland *Correspondence: [email protected] Abstract: Background: Naval service can have a significant impact on the wellbeing of seafarers, and the operation of warships is highly dependent on the personnel on board. Nevertheless, there is a lack of knowledge concerning the impact of seafarers’ individual characteristics on their wellbeing in a naval environment. Therefore, the aim of this study was to investigate individual characteristics of the naval personnel that may be associated with the amount of sleepiness, fatigue and stress responses experienced during shift work and irregular working hours in a naval environment. Methods: The study took place on a Finnish Defence Forces’ Navy missile patrol boat on which 18 crewmembers served as study participants. The measurement periods lasted two separate weeks (seven days and six nights) during shift work with two different watchkeeping systems (4:4, 4:4/6:6). The onboard measurements consisted of the Karolinska Sleepiness Scale, salivary stress hormones, cognitive tests (Sustained Attention to Response Task and N-back Task) and heart rate variability. Results: Participants of older ages or with a longer history in naval service were associated with a greater amount of sleepiness, fatigue and stress responses on board. On the contrary, increased physical activity and a higher level of physical fitness, especially standing long jump, were associated with a lower amount of sleepiness, fatigue and fewer stress responses. In addition, an athletic body composition together with a healthy lifestyle may be beneficial, considering the stress responses on board. Conclusion: The present results are well in line with the previous literature regarding shift work and irregular working hours. The results highlight the importance of regular physical activity and good physical fitness during service in the naval environment. Keywords: individual characteristics; individual factors; shift work; watchkeeping; irregular working hours; sleepiness; fatigue; stress; navy 1. Introduction The operation of naval ships demands a capability which is highly dependent on the personnel on board. The naval environment is quite isolated, and naval service can have a significant impact on the physical and psychological wellbeing of seafarers. Thus, it is important to identify the factors which are associated with the wellbeing of the crew in a naval environment. Int. J. Environ. Res. Public Health 2022,19, 13451. https://doi.org/10.3390/ijerph192013451 https://www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2022,19, 13451 2 of 17 In the military, there are usually requirements for the minimum level of physical fitness. Soldiers are generally expected to be physically fit and capable of completing physically demanding tasks. While being physically fit has, in general, been recognized as an important characteristic in the military [1], there is a lack of knowledge concerning the importance of physical fitness in the naval environment. Previous studies have reported the most physically demanding military naval tasks [ 2 , 3 ]. The categories which have been suggested to be physically demanding in the naval environment are: casualty handling, firefighting, damage control and basic transition tasks [ 3 ]. Out of these categories, casualty handling has been suggested to be the single most demanding task in terms of muscle strength and endurance [ 3 ]. In the case of firefighting on board, a prior study suggested that naval personnel must achieve a maximal oxygen uptake (VO 2 max) value of 41 mL/kg/min as an absolute minimum standard regarding endurance performance [ 4 ]. Regarding stress, greater levels of cardiorespiratory and muscular fitness have been associated with lower stress symptoms among normal-weight men [ 5 , 6 ]. Physical activity has also been associated with less subjective psychological stress [ 7 ]. However, there is still a lack of knowledge concerning the importance of physical fitness in terms of fatigue and stress responses during shift work and irregular working hours in a naval environment. Concerning body composition, a higher body mass index (BMI) and body fat percentage (BFP) have been associated with increased levels of stress [ 8 , 9 ]. Obesity is known to increase the risk of certain health problems (e.g., cardiovascular diseases, diabetes, musculoskeletal disorders and some cancers) which can be problematic when they occur on board [ 10 , 11 ]. Obesity is also suggested to increase the danger of being on board because it may be difficult for obese persons to carry out physically demanding tasks in an emergency situation [ 11 ]. Limited influence over the served food quality and easy access to large portions of food have been identified as possible explanations contributing to obesity among seafarers [ 12 , 13 ]. A recent study regarding military personnel in the U.S. reported that in age groups 20 years or older there were more personnel in the overweight or obese category in the navy (67.0%) than in the coast guard (66.6%), army (63.2%), air force (62.9%) or marine corps (57.8%) [9]. The percentage of obese or overweight personnel also increased with age [ 14 ]. A previous study in the U.S. Navy reported that the relative number of overweight personnel was 69% on small submarines, 66% on large submarines and 63% on aircraft carriers [ 15 ]. The study infers that the more confined the vessel, the higher the percentage of overweight crew. Concerning the psychological aspects of a maritime environment, naval service personnel are dealing with the same kind of stress factors as general seafarers [ 16 ]. Working on a merchant ship is a mentally and physically challenging occupation that has the potential to induce even severe psychological distress [ 17 – 19 ]. In particular, separation from family, time pressure at work, long working days and heat in working areas are the most stressful factors for seafarers [20]. It has been recognized that an individual’s personality is related to one’s ability to deal with occupational stress [ 21 ]. In terms of shift work, extraversion has been positively linked and neuroticism negatively linked with shift work tolerance [ 22 ]. It has also been discovered that openness may be a protective factor against burnout [ 23 ]. When assessing the objective level of psychological stress, salivary alfa-amylase (sAA), salivary cortisol (sCor), salivary immunoglobulin A (sIgA) and salivary dehydroepiandrosterone (sDHEA) have been regarded as reasonable markers [ 24 – 27 ]. As a response to psychological stress, sAA, sCor, and sDHEA increase and sIgA decreases [ 24 – 30 ]. Recent research also supports the use of heart rate variability (HRV) as an objective marker in assessing the level of psychological stress [ 31 ]. HRV is defined as the fluctuation of the length of heartbeat intervals, and it can be utilized to indirectly evaluate the changes in the activity of the autonomic nervous system (ANS). Increased work stress has been reported to be associated with a higher heart rate [ 32 ]. A higher level of stress has also been associated with lower HRV during an orthostatic test [33]. In terms of personal sleep deprivation and poor sleep quality, high scores on the Epworth Sleepiness Scale (ESS) and Pittsburgh Sleep Quality Index (PSQI) have been asso- Int. J. Environ. Res. Public Health 2022,19, 13451 3 of 17 ciated with degraded psychomotor vigilance performance in a naval environment [34,35] . Overall, moderate physical exercise is considered to be beneficial regarding sleep quality [ 36 ]. Considering shift work, older workers have been discovered to face more sleep disturbances than younger ones [ 37 ]. Older age has also been associated with increased sleep problems, in general [ 38 , 39 ]. Concerning shift work and especially older workers, a very rapidly forward-rotating shift system has been found to positively affect sleep when compared to a slower backwards-rotating system [ 40 ]. In a naval environment, during more irregular working hours, fixed watchkeeping systems have been considered more beneficial regarding fatigue than rotating systems [ 41 ]. When assessing the amount of subjective sleepiness, the Karolinska Sleepiness Scale (KSS) has been considered a valid marker and it may also represent a potential marker regarding fatigue [ 42 – 45 ]. When assessing cognitive capability, the Sustained Attention to Response Task (SART) has been used to evaluate sustained attention and inhibitory control [ 46 ], whereas the N-Back Task (N-Back) has been considered a useful test regarding working memory [47]. The aim of this study was to investigate individual characteristics of the naval personnel that have associations with the amount of sleepiness, fatigue and stress responses during shift work and irregular working hours in a naval environment. The examined individual characteristics of the participants were: their age, prior time in naval service, subjective level of physical activity, body composition, physical fitness, blood biomarkers and psychological factors. It was hypothesized that greater physical activity and a higher level of physical fitness in addition to a healthy lifestyle and strong mental performance could attenuate stress responses on board. 2. Materials and Methods 2.1. Study Design on Board The onboard measurements took place on a Finnish Defence Forces’ (FDF) Navy missile patrol boat and consisted of two separate study periods. The measurements lasted 2 weeks in total: 1 week (7 days and 6 nights) with a 4:4 watchkeeping system and another week (7 days and 6 nights) with a 4:4/6:6 watchkeeping system. The structures of the watchkeeping systems are shown in Figures 1and 2. Both watchkeeping systems consisted of two watch sections and, therefore, required two working groups to maintain the systems. When one working group was working, the second working group was resting and the other way around. This meant that both working groups were working a total of 12 h each day. The 4:4 watchkeeping system contained two daily 2 h half-watches (16:00–18:00 h and 18:00–20:00 h) and had a rotating watch schedule. Because of the rotating watch schedule, the working hours during this 4:4 watchkeeping system were completely irregular between the consecutive days. The 4:4/6:6 watchkeeping system contained the same two daily half-watches (16:00–18:00 h and 18:00–20:00 h) and was a fixed system. Because of the fixed watch schedule, the working periods were at the same time each day. Most of the participants were the same in both study periods, and there were 16 days between the periods to make sure that the participants had sufficient time to recover from the first study week. The standing watch duties of the study participants consisted of tasks that did not require any notable physical effort. The state of the sea was similar in both study periods, during the first period mean wind speed was 5.6 m/s (range 0–12 m/s) and during the second period, it was 7.4 m/s (range 2–12 m/s). During the first study period, the missile patrol boat monitored and secured Finland’s territorial integrity and during the second study period it operated in a naval exercise. The onboard measurements consisted of the Karolinska Sleepiness Scale (KSS), salivary alfa-amylase (sAA), salivary cortisol (sCor), salivary immunoglobulin A (sIgA), salivary dehydroepiandrosterone (sDHEA), cognitive tests (SART and N-Back) and heart rate variability (HRV) during an orthostatic test. Int. J. Environ. Res. Public Health 2022,19, 13451 4 of 17 Int. J. Environ. Res. Public Health 2022, 19, x 4 of 18 Figure 1. The structure of the two Section 4:4 watchkeeping system during the first study period. Figure 2. The structure of the two Section 4:4/6:6 watchkeeping system during the second study period. The standing watch duties of the study participants consisted of tasks that did not require any notable physical effort. The state of the sea was similar in both study periods, during the first period mean wind speed was 5.6 m/s (range 0‒12 m/s) and during the second period, it was 7.4 m/s (range 2‒12 m/s). During the first study period, the missile patrol boat monitored and secured Finland’s territorial integrity and during the second study period it operated in a naval exercise. The onboard measurements consisted of the Karolinska Sleepiness Scale (KSS), salivary alfa-amylase (sAA), salivary cortisol (sCor), Figure 1. The structure of the two Section 4:4 watchkeeping system during the first study period. Int. J. Environ. Res. Public Health 2022, 19, x 4 of 18 Figure 1. The structure of the two Section 4:4 watchkeeping system during the first study period. Figure 2. The structure of the two Section 4:4/6:6 watchkeeping system during the second study period. The standing watch duties of the study participants consisted of tasks that did not require any notable physical effort. The state of the sea was similar in both study periods, during the first period mean wind speed was 5.6 m/s (range 0‒12 m/s) and during the second period, it was 7.4 m/s (range 2‒12 m/s). During the first study period, the missile patrol boat monitored and secured Finland’s territorial integrity and during the second study period it operated in a naval exercise. The onboard measurements consisted of the Karolinska Sleepiness Scale (KSS), salivary alfa-amylase (sAA), salivary cortisol (sCor), Figure 2. The structure of the two Section 4:4/6:6 watchkeeping system during the second study period. 2.2. Participant Characteristics In total, 18 healthy male FDF Navy soldiers (n = 14) and conscripts (n = 4) took part in the study. During the first study week (4:4 watchkeeping system), 17 participants (13 FDF Navy soldiers and 4 conscripts) were studied, while during the second week (4:4/6:6 watchkeeping system), 16 participants (13 FDF Navy soldiers and 3 conscripts) were studied. Fifteen participants were the same in both study periods (12 FDF Navy soldiers and 3 conscripts). The participants were recruited from the same FDF Navy missile patrol boat class where the measurements were conducted. They gave their informed consent for the study, participated voluntarily and did not receive any financial gain for their participation in the study. The participants were advised to maintain a regular sleep–wake rhythm and Int. J. Environ. Res. Public Health 2022,19, 13451 5 of 17 avoid sleep deprivation 3 days prior to the measurement periods. They also reported an approximation of the mean duration of their sleep for the last 3 days and the actual duration of their sleep on the last day before the study periods. The FDF Navy soldiers that work in naval duty must be clinically examined every second year before the age of 40 years and every year after the age of 40. Additionally, the conscripts must be clinically examined before naval duty. The examination is performed by a military physician that is an approved medical examiner for seafarers. During this clinical examination, the soldiers are also screened for sleep apnea and should not have any notable sleep problems. All participants that were clinically fit to work in a naval environment were included in the study. The participants’ age, prior time in naval service and subjective level of physical activity were determined at the beginning of the study periods. The body composition, physical fitness and blood biomarkers were determined within 3 months of the onboard study periods. The psychological factors of the participants were determined within 1 year of the onboard study periods. Age and prior time in the naval service were defined in years. The level of subjective physical activity was defined as a numeric value from 0 to 10 according to the physical activity classification shown in Figure 3. Regarding the body composition, the body mass index (BMI), body fat percentage (BFP), skeletal muscle mass (SMM) and fat mass (FATM) were measured in the morning after 10 h of fasting using a segmental multifrequency bioimpedance analysis (InBody 720, Biospace, Seoul, South Korea). The waist circumference (WC) of the participants was also measured. Int. J. Environ. Res. Public Health 2022, 19, x 5 of 18 salivary immunoglobulin A (sIgA), salivary dehydroepiandrosterone (sDHEA), cognitive tests (SART and N-Back) and heart rate variability (HRV) during an orthostatic test. 2.2. Participant Characteristics In total, 18 healthy male FDF Navy soldiers (n = 14) and conscripts (n = 4) took part in the study. During the first study week (4:4 watchkeeping system), 17 participants (13 FDF Navy soldiers and 4 conscripts) were studied, while during the second week (4:4/6:6 watchkeeping system), 16 participants (13 FDF Navy soldiers and 3 conscripts) were studied. Fifteen participants were the same in both study periods (12 FDF Navy soldiers and 3 conscripts). The participants were recruited from the same FDF Navy missile patrol boat class where the measurements were conducted. They gave their informed consent for the study, participated voluntarily and did not receive any financial gain for their participation in the study. The participants were advised to maintain a regular sleep–wake rhythm and avoid sleep deprivation 3 days prior to the measurement periods. They also reported an approximation of the mean duration of their sleep for the last 3 days and the actual duration of their sleep on the last day before the study periods. The FDF Navy soldiers that work in naval duty must be clinically examined every second year before the age of 40 years and every year after the age of 40. Additionally, the conscripts must be clinically examined before naval duty. The examination is performed by a military physician that is an approved medical examiner for seafarers. During this clinical examination, the soldiers are also screened for sleep apnea and should not have any notable sleep problems. All participants that were clinically fit to work in a naval environment were included in the study. The participants’ age, prior time in naval service and subjective level of physical activity were determined at the beginning of the study periods. The body composition, physical fitness and blood biomarkers were determined within 3 months of the onboard study periods. The psychological factors of the participants were determined within 1 year of the onboard study periods. Age and prior time in the naval service were defined in years. The level of subjective physical activity was defined as a numeric value from 0 to 10 according to the physical activity classification shown in Figure 3. Regarding the body composition, the body mass index (BMI), body fat percentage (BFP), skeletal muscle mass (SMM) and fat mass (FATM) were measured in the morning after 10 h of fasting using a segmental multifrequency bioimpedance analysis (InBody 720, Biospace, Seoul, South Korea). The waist circumference (WC) of the participants was also measured. Figure 3. The physical activity classification. Figure 3. The physical activity classification. Concerning physical fitness, a number of physical fitness tests were used: the maximal number of push-ups and sit-ups in one minute, a standing long jump and a 12-min running test. The participants were familiar with these physical fitness tests because they belong to basic military training in the FDF. In addition, the maximal voluntary contraction of the upper (MVCupper) and lower (MVClower) extremities were measured with an electromechanical dynamometer manufactured by the University of Jyväskylä (Jyväskylä, Finland). Both measurements were conducted bilaterally and in a sitting position. When taking the MVCupper measurements, the handlebar was maintained at the height of the shoulders with a 90 ◦ angle from the elbows. During the MVClower measurements, the knee and hip angles were maintained at 107 ◦ and 110 ◦ . A seated medicine ball throw (SMBT) was also performed to assess the rapid power production of the upper limbs. The ball (2kg) was thrown with both hands, keeping forearms equivalent to the floor, back against a wall and legs straight. Distance from the landing point to the wall was measured. Int. J. Environ. Res. Public Health 2022,19, 13451 6 of 17 A supervisor observed all tests and also demonstrated the correct techniques for each test prior to conducting the tests. Venous blood samples were collected from the antecubital vein after an overnight 10-h fast. The following biomarkers were analyzed: the fasting plasma glucose (FPG), fasting plasma insulin (FPI), hemoglobin A1c (HbA1c), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), testosterone (TES) and insulin-like growth factor-1 (IGF-1). The IGF-1 samples were analyzed by the Bioanalytical Laboratory Unit of the Faculty of Sport and Health Sciences (Jyväskylä, Finland). All other blood biomarkers were analyzed by the Tykslab Operational Division Laboratory of the Intermunicipal Hospital District of Southwest Finland. The psychological measures of this study were the Finnish version of the Short Five personality test [ 48 ] and the Finnish version of the shortened resilience scale [ 49 ]. The Short Five personality test (S5) is a shortened version of the Big Five personality test. It evaluates personality by measuring the five main personality traits: extraversion, neuroticism, openness, agreeableness and conscientiousness [ 48 ]. RS14 is a shortened version of the Resilience Scale (RS) and is a valid tool for measuring resilience [ 49 ]. A more detailed description of the individual characteristics of the study participants is shown in Table 1. Table 1. Description of the individual characteristics of the study participants. Range, mean and standard deviation (SD) are presented. FIRST STUDY PERIOD SECOND STUDY PERIOD n Range Mean (SD) n Range Mean (SD) Age (y) 17 19–45 29 (8) 16 19–45 29 (9) Prior time in naval service (y) 17 0.5–22.0 4.2 (5.4) 16 0.5–22.0 4.8 (5.6) Physical activity classification (no.) 17 1–8 6 (2) 16 1–8 6 (2) Body composition Height (m) 16 1.65–1.87 1.79 (0.05) 15 1.65–1.87 1.80 (0.05) Body mass (kg) 16 62.8–102.1 82.8 (8.7) 15 62.8–102.1 83.8 (8.4) Body mass index (kg/m2)16 19.9–32.0 25.9 (3.0) 15 19.9–32.0 25.9 (3.0) Body fat percentage (%) 16 7.7–34.4 20.0 (6.8) 15 7.7–34.4 20.3 (7.0) Skeletal muscle mass (kg) 16 32.8–42.8 37.5 (3.1) 15 32.8–42.8 37.7 (3.0) Fat mass (kg) 16 4.8–31.2 16.9 (6.9) 15 4.8–31.2 17.4 (7.1) Waist circumference (cm) 11 67–113 91 (12) 12 67–113 91 (11) Physical fitness Push-ups (reps/min) 17 22–68 41 (11) 16 22–68 40 (11) Sit-ups (reps/min) 17 22–62 45 (10) 16 22–62 44 (10) Standing long jump (m) 17 1.95–2.68 2.29 (0.19) 16 1.95–2.68 2.29 (0.19) 12-min run test (m) 17 2030–3010 2628 (293) 16 2030–3010 2634 (277) MVCupper (kg) 11 84–124 106 (11) 12 81–124 104 (12) MVClower (kg) 11 323–531 444 (59) 12 292–531 432 (71) SMBT (m) 11 5.10–8.02 6.57 (0.80) 12 5.10–8.02 6.50 (0.80) Blood biomarkers FPG (mmol/l) 11 5.2–6.1 5.5 (0.2) 12 5.0–6.1 5.5 (0.3) FPI (mU/l) 11 5.0–17.0 8.3 (4.0) 12 5.0–17.0 8.5 (3.9) HbA1c (mmol/mol) 11 29.0–36.0 31.9 (2.0) 12 29.0–36.0 32.2 (2.0) TC (mmol/l) 11 3.2–6.3 4.8 (0.9) 12 3.2–6.3 4.8 (0.9) LDL-C (mmol/l) 11 1.2–4.6 3.0 (0.9) 12 1.2–4.6 3.0 (0.9) HDL-C (mmol/l) 11 1.1–2.0 1.5 (0.3) 12 1.1–2.0 1.5 (0.3) TES (nmol/l) 11 8.7–24.0 15.7 (4.8) 12 8.7–24.0 15.4 (4.7) IGF-1 (nmol/l) 11 11.2–23.8 18.0 (4.2) 12 11.2–23.8 18.2 (4.1) Int. J. Environ. Res. Public Health 2022,19, 13451 7 of 17 Table 1. Cont. FIRST STUDY PERIOD SECOND STUDY PERIOD n Range Mean (SD) n Range Mean (SD) Psychological factors S5: Extraversion (no.) 11 −12–12 3 (10) 11 −21–12 1 (11) S5: Neuroticism (no.) 11 −31–3 −17 (10) 11 −29–3 −15 (9) S5: Openness (no.) 11 −6–31 10 (12) 11 −6–31 11 (11) S5: Agreeableness (no.) 11 −4–26 12 (10) 11 −4–26 11 (10) S5: Conscientiousness (no.) 11 4–32 19 (9) 11 4–27 17 (8) RS14 (no.) 11 62–95 79 (11) 11 62–94 77 (9) Before measurement periods Sleep/day, last 3 days (h) 16 5.5–8.0 7.0 (0.7) 15 6.0–8.8 7.5 (0.7) Sleep/day, last 1 day (h) 16 6.5–9.0 7.4 (0.7) 15 5.0–10.0 7.0 (0.8) Note. MVCupper = maximal voluntary contraction of the upper extremities, MVClower = maximal voluntary contraction of the lower extremities, SMBT = seated medicine ball throw, FPG = fasting plasma glucose, FPI = fasting plasma insulin , HbA1c = hemoglobin A1c, TC = total cholesterol, LDL-C = low-density lipoprotein cholesterol, HDL-C = high-density lipoprotein cholesterol, TES = testosterone, IGF-1 = insulin-like growth factor-1, S5 = the Short Five Personality test, RS14 = shortened version of the Resilience Scale. 2.3. Measurements on Board the Navy Missile Patrol Boat 2.3.1. The Karolinska Sleepiness Scale (KSS) During study periods, the participants reported numeric KSS values in individual sleep diaries at the beginning and end of each watch. The scale consisted of values from 1 (extremely alert) to 9 (extremely sleepy or fighting sleep). 2.3.2. Salivary AA, Cor, IgA and DHEA Saliva samples were collected daily at 16:00 h or 18:00 h depending on the watchkeeping system with a Salivette ® sampling device in accordance with the instructions of the device. The participants did not use any cortisol medication during or before the measurements. They were advised to avoid physical exercise for 3 h and to avoid brushing their teeth or eating for 1 h before giving the saliva sample. The collected saliva samples were stored in a freezer before further analysis. After defrosting, sAA, sCor, sIgA and sDHEA were analyzed by the Bioanalytical Laboratory Unit of the Faculty of Sport and Health Sciences (Jyväskylä, Finland). 2.3.3. Orthostatic Test After the collection of the saliva samples, an orthostatic test, with 5 min supine (SU) and 5 min standing (ST), was performed as a daily measurement point for the HRV measures. All study participants on the same watch section performed the orthostatic test at the same time and were advised to avoid all other physical activities while completing the test. 2.3.4. Cognitive Tests (SART, N-Back) After completing the orthostatic test, participants performed SART and N-Back with laptop computers. New, unused keyboards were attached to all computers and the participants completed the tests using the same keyboard and computer on all measurements. A more precise conduction of the cognitive tests is described in a prior study [50]. In the present study, the mean reaction times in the correct response trials (SART RT) and the number of commission errors (SART Errors) were evaluated using SART. In the N-Back, the number of correct responses (N-Back Total hits) and the number of commission errors (N-Back Errors) were evaluated. Int. J. Environ. Res. Public Health 2022,19, 13451 8 of 17 2.3.5. Heart Rate Variability (HRV) The HRV was recorded with the Bodyguard 2 device (Firstbeat Technologies Ltd., Jyväskylä, Finland) which recorded R to R intervals at a sampling frequency of 1000 Hz. The HRV during an orthostatic test was recorded daily, except on the fourth day of both study periods because the HRV measurement device had to be recharged. Regarding the analysis, there were 1-min baseline recordings in the SU and ST positions so that, in both positions, only the last 4 min of HRV were used for analysis to obtain reliable results [ 51 ]. The analysis was performed using the Kubios HRV Standard program (version 3.4.3, Kubios Ltd., Kuopio, Finland). In the present study, the mean heart rate (HRmean), the standard deviation of NN intervals (SDNN), root mean square of successive RR interval differences (RMSSD), absolute total power (TP), the absolute power of the very low-frequency band (VLF), absolute power of the low-frequency band (LF), absolute power of the high-frequency band (HF), and the ratio of LF to HF power (LF/HF) were investigated. A recent study assessing stress during an orthostatic test pointed out that a higher level of stress was associated with a lower HRV overall [ 33 ]. Still, a frequently reported finding in terms of HRV and stress is low parasympathetic activity, which is perceived as a decrease in HF and an increase in LF power [ 31 ]. Despite the fact that in long-term ambulatory recordings the LF power has been reported to increase with sympathetic activity [ 52 ], this does not seem to occur in short-term resting recordings [ 53 ]. In short-term resting recordings, the LF power has been reported to increase with slower breathing and it is almost an entirely vagally mediated parasympathetic activity [ 53 ]. A high LF/HF ratio has also been considered a controversial marker for sympathetic activity and should be used with caution in short-term recordings [ 53 ]. In the present study, an elevated HRmean, lower HRV overall and higher LF/HF ratio were considered stress responses during the orthostatic test. 2.4. Statistics Statistical analysis was performed using the SPSS statistical software (SPSS version 27.0.1.0; SPSS Inc., Chicago, IL, USA). A Shapiro–Wilk test was used to assess the normal distribution of the data. Correlations between normally distributed variables were calculated using a Pearson correlation coefficient, and a Spearman correlation coefficient was used for non-normally distributed variables. Correlations between the individual characteristics and KSS were calculated using the mean or median of absolute KSS values. All other correlations between the individual characteristics and onboard measurements were conducted using the mean or median of absolute differences between the baseline values (first measured values) and follow-up values (later measured values) of the onboard measurements. Any p-values lower than 0.05 were considered statistically significant. 3. Results 3.1. The Most Distinct Associations between Sleepiness, Fatigue, Stress Responses and Individual Characteristics The participants’ age and prior time in naval service had the most distinct positive associations with the amount of sleepiness and fatigue and the stress responses during the study weeks. In contrast, their physical activity classification and standing long jump had negative associations. Regarding sleepiness and fatigue, the correlations between these individual characteristics and KSS scores overall are displayed in Figure 4. Significant positive associations with KSS scores overall during both study weeks were observed regarding age (1. r = 0.52, p= 0.03 and 2. r = 0.58, p= 0.02) and prior time in the naval service (1. r = 0.74, p< 0.01 and 2. r = 0.73, p< 0.01), whereas significant negative associations with KSS scores overall were observed regarding physical activity (1. r = − 0.61, p< 0.01 and 2. r = −0.55, p= 0.03) and standing long jump (2. r = −0.54, p= 0.03). Int. J. Environ. Res. Public Health 2022,19, 13451 15 of 17 Institutional Review Board Statement: The study was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Tampere University Hospital (R18166/2018). Research permission was granted from the Finnish Defence Forces’ Defence Command (AO22851, 18 December 2018). Informed Consent Statement: Written informed consent was obtained from all subjects involved in the study. Data Availability Statement: The datasets used and analyzed during the current study are the property of the Navy Command Finland. All data is primarily not public, but the data availability can be sought from the corresponding author on reasonable request. Acknowledgments: We thank the crewmembers of the FDF Navy missile patrol boat who participated in the study. Our thanks also goes to the staff of the Finnish Defence Research Agency’s Human Performance Division and Coastal Fleet for their help performing the study. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. References 1. Kyröläinen, H.; Häkkinen, K.; Kautiainen, H.; Santtila, M.; Pihlainen, K.; Häkkinen, A. Physical fitness, BMI and sickness absence in male military personnel. Occup. Med. 2008,58, 251–256. [CrossRef] 2. Middleton, K.J.; Carr, A.J. The identification of combat survivability tasks associated with naval vessel damage in maritime environments. Appl. Ergon. 2017,59, 27–33. [CrossRef] 3. Brown, P.; Fallowfield, J.L. Physical Employment Standards for UK Royal Navy Personnel: A Survey of Tasks That Require Muscle Strength and Endurance. Mil. Med. 2019,184, 882–888. [CrossRef] [PubMed] 4. Bilzon, J.L.; Scarpello, E.G.; Smith, C.V.; Ravenhill, N.A.; Rayson, M.P. Characterization of the metabolic demands of simulated shipboard Royal Navy fire-fighting tasks. Ergonomics 2001,44, 766–780. [CrossRef] 5. Kettunen, O.; Kyröläinen, H.; Santtila, M.; Vasankari, T. Physical fitness and volume of leisure time physical activity relate with low stress and high mental resources in young men. J. Sports Med. Phys. Fitness 2014,54, 545–551. 6. Kettunen, O.; Kyröläinen, H.; Santtila, M.; Vuorimaa, T.; Vasankari, T.J. Greater levels of cardiorespiratory and muscular fitness are associated with low stress and high mental resources in normal but not overweight men. BMC Public Health 2016 ,16, 788. [CrossRef] [PubMed] 7. Stults-Kolehmainen, M.A.; Sinha, R. The effects of stress on physical activity and exercise. Sports Med. 2014 ,44, 81–121. [CrossRef] [PubMed] 8. Teisala, T.; Mutikainen, S.; Tolvanen, A.; Rottensteiner, M.; Leskinen, T.; Kaprio, J.; Kolehmainen, M.; Rusko, H.; Kujala, U.M. Associations of physical activity, fitness, and body composition with heart rate variability-based indicators of stress and recovery on workdays: A cross-sectional study. J. Occup. Med. Toxicol. 2014,9, 16. [CrossRef] 9. Mujica-Parodi, L.R.; Renelique, R.; Taylor, M.K. Higher body fat percentage is associated with increased cortisol reactivity and impaired cognitive resilience in response to acute emotional stress. Int. J. Obes. 2009,33, 157–165. [CrossRef] 10. WHO Obesity and Overweight. Available online: https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight (accessed on 19 August 2022). 11. Hoeyer, J.L.; Hansen, H.L. Obesity among Danish seafarers. Int. Marit. Health 2005,56, 48–55. 12. Hansen, H.L.; Hjarnoe, L.; Jepsen, J.R. Obesity continues to be a major health risk for Danish seafarers and fishermen. Int. Marit. Health 2011,62, 98–103. [PubMed] 13. Oldenburg, M.; Harth, V.; Jensen, H.J. Overview and prospect: Food and nutrition of seafarers on merchant ships. Int. Marit. Health 2013,64, 191–194. [CrossRef] [PubMed] 14. 2018 Department of Defense Health Related Behaviors Survey (HRBS). Available online: https://www.rand.org/pubs/research_ reports/RR4222.html (accessed on 19 August 2022). 15. Gregg, M.A.; Jankosky, C.J. Physical readiness and obesity among male U.S. Navy personnel with limited exercise availability while at sea. Mil. Med. 2012,177, 1302–1307. [CrossRef] 16. Sanden, S.; Johnsen, B.H.; Eid, J.; Sommerfelt-Pettersen, J.; Koefoed, V.; Størksen, R.; Røsseland, A.; Neteland, H.O.; Wetteland, P.I.; Wilhelmsen, E.V. Mental readiness for maritime international operation: Procedures developed by Norwegian navy. Int. Marit. Health 2014,65, 93–97. [CrossRef] 17. Allen, P.; Wadsworth, E.; Smith, A. Seafarers’ fatigue: A review of the recent literature. Int. Marit. Health 2008 ,59, 81–92. [PubMed] 18. Wadsworth, E.J.; Allen, P.H.; McNamara, R.L.; Smith, A.P. Fatigue and health in a seafaring population. Occup. Med. 2008 ,58, 198–204. [CrossRef] 19. Roberts, S.E.; Jaremin, B.; Chalasani, P.; Rodgers, S.E. Suicides among seafarers in UK merchant shipping, 1919–2005. Occup. Med. 2010,60, 54–61. [CrossRef] Int. J. Environ. Res. Public Health 2022,19, 13451 16 of 17 20. Oldenburg, M.; Jensen, H.J.; Latza, U.; Baur, X. Seafaring stressors aboard merchant and passenger ships. Int. J. Public Health 2009 , 54, 96–105. [CrossRef] 21. Kenny, D.T.; Cooper, C.L. Introduction: Occupational stress and its management. Int. J. Stress Manag. 2003 ,10, 275–279. [CrossRef] 22. Ritonja, J.; Aronson, K.J.; Matthews, R.W.; Boivin, D.B.; Kantermann, T. Working Time Society consensus statements: Individual differences in shift work tolerance and recommendations for research and practice. Ind. Health 2019 ,57, 201–212. [CrossRef] [PubMed] 23. De la Fuente-Solana, E.I.; Suleiman-Martos, N.; Velando-Soriano, A.; Cañadas-De la Fuente, G.R.; Herrera-Cabrerizo, B.; AlbendínGarcía, L. Predictors of burnout of health professionals in the departments of maternity and gynaecology, and its association with personality factors: A multicentre study. J. Clin. Nurs. 2021,30, 207–216. [CrossRef] [PubMed] 24. Strahler, J.; Mueller, A.; Rosenloecher, F.; Kirschbaum, C.; Rohleder, N. Salivary alpha-amylase stress reactivity across different age groups. Psychophysiology 2010,47, 587–595. [CrossRef] 25. Hellhammer, D.H.; Wüst, S.; Kudielka, B.M. Salivary cortisol as a biomarker in stress research. Psychoneuroendocrinology 2009 ,34, 163–171. [CrossRef] 26. Afrisham, R.; Aberomand, M.; Soliemanifar, O.; Kooti, W.; Ashtary-Larky, D.; Alamiri, F.; Najjar-Asl, S.; Khaneh-Keshi, A.; Sadegh-Nejadi, S. Levels of salivary immunoglobulin A under psychological stress and its relationship with rumination and five personality traits in medical students. Eur. J. Psychiat. 2016,30, 41–53. 27. Izawa, S.; Sugaya, N.; Shirotsuki, K.; Yamada, K.C.; Ogawa, N.; Ouchi, Y.; Nagano, Y.; Suzuki, K.; Nomura, S. Salivary dehydroepiandrosterone secretion in response to acute psychosocial stress and its correlations with biological and psychological changes. Biol. Psychol. 2008,79, 294–298. [CrossRef] [PubMed] 28. Deinzer, R.; Kleineidam, C.; Stiller-Winkler, R.; Idel, H.; Bachg, D. Prolonged reduction of salivary immunoglobulin A (sIgA) after a major academic exam. Int. J. Psychophysiol. 2000,37, 219–232. [CrossRef] 29. Jemmott, J.B.; Magloire, K. Academic stress, social support, and secretory immunoglobulin A. J. Pers. Soc. Psychol. 1988 ,55, 803. [CrossRef] [PubMed] 30. Lowe, G.; Urquhart, J.; Greenman, J.; Lowe, G. Academic stress and secretory immunoglobulin A. Psychol. Rep. 2000 ,87, 721–722. [CrossRef] 31. Kim, H.G.; Cheon, E.J.; Bai, D.S.; Lee, Y.H.; Koo, B.H. Stress and heart rate variability: A meta-analysis and review of the literature. Psychiatry Investig. 2018,15, 235–245. [CrossRef] 32. Vrijkotte, T.G.; van Doornen, L.J.; de Geus, E.J. Effects of work stress on ambulatory blood pressure, heart rate, and heart rate variability. Hypertension 2000,35, 880–886. [CrossRef] 33. Hynynen, E.; Konttinen, N.; Kinnunen, U.; Kyröläinen, H.; Rusko, H. The incidence of stress symptoms and heart rate variability during sleep and orthostatic test. Eur. J. Appl. Physiol. 2011,111, 733–741. [CrossRef] [PubMed] 34. Shattuck, N.L.; Matsangas, P. Psychomotor vigilance performance predicted by Epworth Sleepiness Scale scores in an operational setting with the United States Navy. J. Sleep Res. 2015,24, 174–180. [CrossRef] [PubMed] 35. Matsangas, P.; Shattuck, N.L. Sleep quality, occupational factors, and psychomotor vigilance performance in the U.S. Navy sailors. Sleep 2020,43, zsaa118. [CrossRef] [PubMed] 36. Wang, F.; Boros, S. The effect of physical activity on sleep quality: A systematic review. Eur. J. Physiother. 2021 ,23, 11–18. [CrossRef] 37. Hulsegge, G.; Loef, B.; van Kerkhof, L.W.; Roenneberg, T.; van der Beek, A.J.; Proper, K.I. Shift work, sleep disturbances and social jetlag in healthcare workers. J. Sleep Res. 2019,28, e12802. [CrossRef] 38. Reinberg, A.; Ashkenazi, I. Internal desynchronization of circadian rhythms and tolerance to shift work. Chronobiol. Int. 2008 ,25, 625–643. [CrossRef] 39. Gander, P.; O’Keeffe, K.; Santos-Fernandez, E.; Huntington, A.; Walker, L.; Willis, J. Fatigue and nurses’ work patterns: An online questionnaire survey. Int. J. Nurs. Stud. 2019,98, 67–74. [CrossRef] 40. Härmä, M.; Hakola, T.; Kandolin, I.; Sallinen, M.; Virkkala, J.; Bonnefond, A.; Mutanen, P. A controlled intervention study on the effects of a very rapidly forward rotating shift system on sleep-wakefulness and well-being among young and elderly shift workers. Int. J. Psychophysiol. 2006,59, 70–79. [CrossRef] 41. Arulanandam, S.; Tsing, G.C. Comparison of alertness levels in ship crew. An experiment on rotating versus fixed watch schedules. Int. Marit. Health 2009,60, 6–9. 42. Akerstedt, T.; Gillberg, M. Subjective and objective sleepiness in the active individual. Int. J. Neurosci. 1990 ,52, 29–37. [CrossRef] 43. Akerstedt, T.; Knutsson, A.; Westerholm, P.; Theorell, T.; Alfredsson, L.; Kecklund, G. Mental fatigue, work and sleep. J. Psychosom. Res. 2004,57, 427–433. [CrossRef] [PubMed] 44. Kaida, K.; Takahashi, M.; Akerstedt, T.; Nakata, A.; Otsuka, Y.; Haratani, T.; Fukasawa, K. Validation of the Karolinska sleepiness scale against performance and EEG variables. Clin. Neurophysiol. 2006,117, 1574–1581. [CrossRef] [PubMed] 45. Lutzhoft, M.; Dahlgren, A.; Kircher, A.; Thorslund, B.; Gillberg, M. Fatigue at sea in Swedish shipping: A field study. Am. J. Ind. Med. 2010,53, 733–740. [CrossRef] [PubMed] 46. Robertson, I.H.; Manly, T.; Andrade, J.; Baddeley, B.T.; Yiend, J. ‘Oops!’: Performance correlates of everyday attentional failures in traumatic brain injured and normal subjects. Neuropsychologia 1997,35, 747–758. [CrossRef] 47. Jaeggi, S.M.; Buschkuehl, M.; Perrig, W.J.; Meier, B. The concurrent validity of the N-back task as a working memory measure. Memory 2010,18, 394–412. [CrossRef] Int. J. Environ. Res. Public Health 2022,19, 13451 17 of 17 48. Konstabel, K.; Lönnqvist, J.E.; Walkowitz, G.; Konstabel, K.; Verkasalo, M. The ‘Short Five’ (S5): Measuring personality traits using comprehensive single items. Eur. J. Personal. 2012,26, 13–29. [CrossRef] 49. Losoi, H.; Turunen, S.; Wäljas, M.; Helminen, M.; Öhman, J.; Julkunen, J.; Rosti-Otajärvi, E. Psychometric Properties of the Finnish Version of the Resilience Scale and its Short Version. Psychol. Community Health 2013,2, 1–10. [CrossRef] 50. Myllylä, M.; Kyröläinen, H.; Ojanen, T.; Ruohola, J.P.; Heinonen, O.J.; Vahlberg, T.; Parkkola, K.I. Effects of operational assessment of the 4:4 and 4:4/6:6 watch systems on sleepiness, fatigue, and stress responses during patrolling on a navy missile patrol boat. Chronobiol. Int. 2022,39, 1233–1241. [CrossRef] 51. Bourdillon, N.; Schmitt, L.; Yazdani, S.; Vesin, J.M.; Millet, G.P. Minimal Window Duration for Accurate HRV Recording in Athletes. Front. Neurosci. 2017,11, 456. [CrossRef] 52. Axelrod, S.; Lishner, M.; Oz, O.; Bernheim, J.; Ravid, M. Spectral analysis of fluctuations in heart rate: An objective evaluation of autonomic nervous control in chronic renal failure. Nephron 1987,45, 202–206. [CrossRef] 53. Shaffer, F.; McCraty, R.; Zerr, C.L. A healthy heart is not a metronome: An integrative review of the heart’s anatomy and heart rate variability. Front. Psychol. 2014,5, 1040. [CrossRef] [PubMed] 54. Härmä, M.; Ilmarinen, J.; Knauth, P.; Rutenfranz, J.; Hänninen, O. Physical training intervention in female shift workers: I. The effects of intervention on fitness, fatigue, sleep, and psychosomatic symptoms. Ergonomics 1988 ,31, 39–50. [CrossRef] [PubMed] 55. Kim, K.; Park, S.M. Association of muscle mass and fat mass with insulin resistance and the prevalence of metabolic syndrome in Korean adults: A cross-sectional study. Sci. Rep. 2018,8, 2703. [CrossRef] [PubMed] 56. Nykänen, T.; Pihlainen, K.; Kyröläinen, H.; Fogelholm, M. Associations of nutrition and body composition with cardiovascular disease risk factors in soldiers during a 6-month deployment. Int. J. Occup. Med. Environ. Health 2020,33, 457–466. [CrossRef] 57. Mudali, S.; Dobs, A.S. Effects of testosterone on body composition of the aging male. Mech. Ageing Dev. 2004 ,125, 297–304. [CrossRef] 58. Crichton, G.E.; Elias, M.F.; Davey, A.; Sullivan, K.J.; Robbins, M.A. Higher HDL cholesterol is associated with better cognitive function: The Maine-Syracuse study. J. Int. Neuropsychol. Soc. 2014,20, 961–970. [CrossRef] 59. Jackson, J.J.; Thoemmes, F.; Jonkmann, K.; Lüdtke, O.; Trautwein, U. Military training and personality trait development: Does the military make the man, or does the man make the military? Psychol. Sci. 2012,23, 270–277. [CrossRef]